Substantial prior art is available in the art of insulators including insulators useful for high voltage applications. Such insulators have been produced from a variety of compositions including the use of ethylene propylene rubber, as well as ethylene propylene diene terpolymer compositions or the so-called EPDM polymers. For example, U.S. Pat. Nos. 3,878,319; 3,935,042; 4,125,509 and 4,133,936 describe insulators for electrical applications such as power cables which comprise a thermoset ethylene propylene rubber.
It is known from Luh, D. R., "Ethylene Propylene Terpolymers in Wire and Cable Constructions," Wire & Wire Products, 4-70, pp. 79-81 (1970), that EPDM terpolymers provide a highly useful combination of characteristics which makes such polymers desirable in cable construction. For example, the polymers have excellent resistance to the deteriorating effects of ozone, oxygen, weather, heat and many chemicals, while also having good mechanical properties and excellent electrical and low temperature characteristics.
Ethylene propylene terpolymers (designated EPDM by the American Society for Testing and Materials) have been synthesized using a variety of dienes. One method of preparation involves the terpolymers of ethylene, propylene, and 1,4-hexadiene. The main polymer chain is completely saturated and consists of randomly arranged ethylene and propylene units with pendant diene units spaced at less frequent intervals. These terpolymers offer broad curing versatility; thus vulcanization can be accomplished either by using sulfur-based systems that permit cross-linking through the pendant double bond, or by using organic peroxides that develop crosslinks through both the pendant double bond and the main polymer chain. Radiation may be used.
While the ethylene propylene terpolymers provide a highly desirable combination of excellent resistance to the deteriorating effects of ozone, oxygen, weather, heat and many chemicals, with good mechanical properties and excellent electrical and low temperature characteristics, they do have the disadvantage that being hydrocarbon products, they are not inherently resistant to flame. The present invention provides EPDM products which overcome this disadvantage.
EPDM terpolymers have also been used in various flame resistant compositions. Thus, U.S. Pat. No. 4,125,509 discloses ethylene propylene rubber compositions having flame resistant properties, which compositions contain chlorosulfonated polyethylene, zinc oxide, hydrated alumina and amorphous silica. U.S. Pat. No. 4,211,692 discloses coating compositions for polyolefin substrates which comprise EPDM terpolymer rubber, a high density polyethylene and ethylene-propylene block copolymer, and an ethylene-isobutyl-acrylate copolymer, together with optional stabilizers, flame retardants, and pigments. The fire retardant compositions in this patent comprise one or more polyhalogenated organic compounds and antimony trioxide. U.S. Pat. No. 4,059,561 discloses flame resistant compositions comprising ring-opening polymerization products of norbornene derivatives wherein the flame resistant components comprise halogen-containing aromatic compounds. Metal inorganic compounds such an antimony trioxide and zinc borate may also be present in the composition.
U.S. Pat. Nos. 3,911,202; 4,001,128 and 4,096,351 describe high voltage insulating materials produced from ethylene propylene diene terpolymers which are available commercially. The above described patents provide insulator products resulting from EPDM materials which have various characteristics with respect to elongation, hardness, tensile strength and the like.
To the applicants' knowledge, however, none of these prior art materials have suitable fire retardant characteristics so as to be used commercially in high voltage applications for electrical use. The present invention provides an EPDM composition which has outstanding flame retardant characteristics, excellent tensile strength, elongation and hardness and good flow during the molding process.